Within the context of the invention, operating fluid containers made of thermoplastic are in particular, but not exclusively, fuel containers for motor vehicles, wiping water containers, oil containers, secondary fluid containers or additive containers for motor vehicles. Containers of the type mentioned at the beginning are frequently produced by extrusion blow molding, wherein in particular high density polyethylene (HDPE) is suitable for the production of extrusion-blow-molded containers.
In the case of motor vehicles having an internal combustion engine, when an operating fluid container, in particular the fuel container, is subjected to heat, the operating fluid, e.g. the fuel, is likewise heated, such that the vapor pressure of the operating fluid increases and the operating fluid container is subjected to a corresponding internal pressure, as a result of which the fuel container is subject to deformation.
For the ventilation of an operating fluid container in the form of a fuel container, said container is fluidically connected to an activated carbon filter for filtering out fuel vapors. The activated carbon filter is flushed by means of intake air during operation of the internal combustion engine, such that fuel vapors bound in the activated carbon can be fed to the internal combustion engine. The absorption capacity of the activated carbon filter can be limited on account of the flushing process using intake air.
In the case of hybrid motor vehicles, there is furthermore a further problem caused by the reduced operating time of the internal combustion engine. On account of the reduced operating time of the internal combustion engine, an activated carbon filter which is fluidically connected to the fuel container undergoes correspondingly less flushing, and therefore also less fuel vapor bound in the activated carbon can be flushed out. This in turn has the effect that activated carbon filters for hybrid motor vehicles have to have larger dimensions. Furthermore, through the ventilation of the fuel container via the activated carbon filter, more fuel is converted into the vapor phase, and therefore it appears to be advantageous to design the fuel container to be stiffer and/or more pressure-resistant.
It is known from the prior art to stiffen the fuel container using winding configurations and/or stiffening elements inside the fuel container. However, wound fuel containers are complex in their production and therefore costly. In addition, an effectively windable geometry limits the degree of configuration freedom in the design and therefore the utilizable volume.
In order to introduce a stiffening element into the operating fluid container, it is known from the prior art to use the “twin sheet blowmolding” method for producing the operating fluid container, in which either two sheet-like preforms are extruded from an extrusion head with two extrusion nozzles, or a tubular preform is separated into two sheet-like lobes. The sheet-like preforms are each molded in a blow mold half of a multi-part blow mold by means of differential pressure. Subsequently, by means of an intermediate frame of the blow mold, positioned between the two blow mold halves, a stiffening element is positioned between the molded container shells, which are still located in the cavities of the blow mold halves, of the operating fluid container and connected to at least one inner side of a container shell. Finally, the intermediate frame between the blow mold halves is removed, and, by the container shells being brought together, the stiffening element is connected to a further inner side of a container shell such that the stiffening element can counteract pressure-induced deformation.
Fuel containers, in particular for gasoline, are customarily manufactured from multi-layered, co-extruded plastics. At least one barrier layer for hydrocarbons is embedded in the wall of the fuel container. The operating fluid container produced by means of the “twin sheet blow molding” method therefore affords the advantage that the barrier layer is not damaged during the introduction of the stiffening element, and therefore no leakage of hydrocarbons is formed. However, compared with the blow molding of a tubular preform, the “twin sheet blow molding” method is time-consuming and costly.